One of conventional optically bistable photodiode devices is described on pages 13 to 15 of "Applied Physics Letters" 45(1) published on July 1, 1984. The optically bistable photodiode device comprises a pin photodiode and a constant voltage supply from which a reverse biased voltage is applied through a series resistor across the pin photodiode. The pin photodiode includes a multiple quantum well inside an intrinsic region to which an electric field is applied perpendicularly under the reverse biased voltage.
In operation, a wavelength of the incident light is chosen to be near the exciton resonance position for zero voltage across the photodiode. When an optical power is low, the supply voltage is substantially applied across the photodiode because no voltage is dropped by the series resistor due to little photocurrent so that the exciton absorption is shifted to longer wavelengths. When the optical power is increased, the photocurrent is increased so that a voltage applied across the photodiode is decreased. This results in the increase of the exciton absorption due to the moving back of the exciton resonances whereby the photocurrent is further increased. As a result, an optically bistable switching operation is performed in the pin photodiode device.
Such an optically bistable photodiode device as described above has characteristics of two stable states and non-linear light transmission in regard to optical input and output. Therefore, the photodiode device is considered to be most important in optical arithmetic unit and memory for an optical information processing apparatus in the near future. For this reason, research and development have been and will be repeated on such a photodiode device in many organizations in the world.
A switching speed and power consumption are the most important characteristics for such a photodiode device, especially in an optical digital arithmetic processing apparatus which is one of applicable fields in view of such advantageous features of light as being fast in its velocity and used in parallel without interference. For instance, much attention has been paid to a consuming energy which is a multiplied value of a switching speed and consuming power as one of performances for an optically bistable photodiode devices, and research and development have been continued to reduce such a consuming energy.
In the optically bistable photodiode device as described before, however, a consuming energy is reported to be as high as 1nJ, in spite of the fact that the consuming energy is desired to be as low as 1pJ, even more 1fJ as a target value for a practical use of an optical digital arithmetic processing apparatus.
In this regard, there is a limitation in reducing a consuming energy for the reason why a bias resistor can not be low in its value beyond some extent for the principle of operation in the conventional optically bistable photodiode device, although a response speed can be fast in inversely proportional to a time constant based on the bias resistor and junction capacitance of the photodiode.